This replaces minimal-configuration.nix.
importable-configuration.nix can be directly added to an existing
NixOS configuration. This makes it easy for users to get started quickly.
This change is fully backwards compatible.
We continue to use the standard non-flake evaluation mode in our
examples and internal tooling until the flakes design has stabilized.
'clightning-plugins = pkgs.recurseIntoAttrs' in pkgs/default.nix is
needed by flake-utils.lib.flattenTree in flake.nix.
It transforms the packages in `clightning-plugins` to top-level packages
named like `clightning-plugins/summary`. (The flake attr `packages`
must be a non-nested attrset of derivations.)
